isl_ast_build_expr.c: ast_expr_from_pw_aff: extract out parts of the function
isl_ast_build_expr.c: ast_expr_from_pw_aff: extract out parts of the function

This will make it easier to change the way the expression is constructed
in subsequent commits.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/isl,inducer/isl-mirror,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,BenzoSM/isl,Meinersbur/isl,UBERTC/isl,Meinersbur/isl,nicolasvasilache/isl,inducer/isl-mirror,tobig/isl,BenzoSM/isl,PollyLabs/isl,UBERTC/isl,UBERTC/isl,nicolasvasilache/isl,BenzoSM/isl,inducer/isl-mirror,UBERTC/isl,tobig/isl,tobig/isl,PollyLabs/isl,inducer/isl-mirror,PollyLabs/isl,tobig/isl,Meinersbur/isl,BenzoSM/isl,BenzoSM/isl,nicolasvasilache/isl,Meinersbur/isl,PollyLabs/isl